It is know in the metal die casting industry to employ a gas venting system of the axial melt impingement type. The system includes a valve movable to an open position where the die or mold cavity is in communication with a gas vent passage to the outside or to a closed position where the mold cavity is not in such communication. As the gas venting system is engaged on the mold during normal operation after mold warm-up, the valve is moved to the open position by application of pneumatic pressure to a cylinder in which a valve extension remote from the valve head is slidably received as a piston. After positioning in the open position, the valve is held in the open position mechanically by engagement of spring biased balls against an intermediate stem portion of the valve. Pneumatic pressure in the cylinder is discontinued once the valve is so engaged and held in open position. A molten charge in then injected into the mold cavity with gas in the cavity being vented past the open valve. The valve is closed by impingement of the molten charge on the valve head after gas is substantially vented from the cavity. A return spring assists return of the valve to the closed position after initial melt impingement on the valve head.
A problem occurs when the valve is not in the proper position during critical stages of the casting or molding sequence, as, for example, a valve closed position during mold cavity fill blocks gas from venting.